By messaging existing clients before advertising the opening. A short post or message naming this week's remaining availability fills gaps that would otherwise stay empty, and an existing client is more likely to rebook again afterward.
For a solo practitioner an unfilled hour is gone permanently, which makes gap filling one of the few genuinely direct-response marketing activities in the trade.
Where to go first, in order:
Two practical points. Do not discount to fill a gap as a habit, because regular clients learn to wait for it. And make the message easy to act on with a direct booking link rather than asking people to reply.
A short automated message to overdue clients, running weekly, prevents many of these gaps from opening at all.